Options:

1 - Ignore it and risk a default judgment.

2 - Call the plaintiff (or his attorney if he has one) and arrange to accept service.

3 - File something with the court bringing the error to the court's attention, send copy to plaintiff.

Bottom line, you know you are being sued. If you have any defense at all, you'll need to preserve your right to raise it. If you have no defense, might as well just let it go to default because they'll eventually get a judgment anyway.